The invention relates to a process and a device for manufacturing a composite strand formed by combining a multiplicity of continuous glass filaments with continuous high-shrinkage organic thermoplastic filaments.
The manufacture of composite strands is disclosed especially in EP-A-0 367 661 which describes a process employing a first installation comprising a bushing that contains molten glass, from which continuous glass filaments are drawn, and a second installation comprising a spinning head, supplied under pressure with an organic thermoplastic that delivers continuous filaments.
The two types of filaments are assembled into at least one composite strand and during the assembly the filaments may be in the form of webs, or in web and strand form. In the composite strand, the glass filaments or strand are surrounded by thermoplastic filaments that protect the glass from rubbing on the solid surfaces with which the strand is in contact.
Although the incorporation of thermoplastic filaments enables the abrasion resistance of the strand to be improved, it also introduces tensions in the strand due to a shrinkage phenomenon of said filaments, which causes waviness of the glass filaments. The presence of waviness is particularly visible when the composite strand is wound in the form of a bobbin as this is deformed over its entire periphery.
The shrinkage phenomenon has several drawbacks: it requires resorting to thick spools for producing the bobbins so that they can withstand the shrinking exerted by the composite strand and it disrupts the unwinding of the strand from the bobbin due to the fact that it does not have the ideal geometric characteristics that are required for the desired application. Furthermore, such a strand is not advantageous for producing a fabric that can be used as a reinforcing material for large-size flat parts since, because of the waviness, the filaments are not perfectly aligned in the final composite. The reinforceability of the strands in a given direction is found to be reduced.
To solve the problem of shrinkage of the thermoplastic filaments, various solutions have been proposed.
In EP-A-0 505 275, a process for manufacturing a composite strand similar to that described previously in EP-A-0 367 661 is proposed, which plans to form the thermoplastic filaments using a spinning head that is normally used in the field of the synthetic fibre industry. In this way, it is possible to obtain a composite strand formed from one or more glass strands surrounded by organic filaments, which is independent of the configuration of the spinning head used for extruding the organic filaments.
In EP-A-0 599 695, it is proposed to mingle the thermoplastic filaments with glass filaments at a speed during the commingling that is greater than the drawing speed of the glass filaments. The speed difference is determined so that the shrinkage phenomenon compensates for the excess initial length of the thermoplastic filaments relative to the glass filaments.
In one embodiment, the thermoplastic filaments pass onto a variable speed drawing unit of the type comprising drums, which accentuates the excess length, which makes it possible to obtain a composite strand of which the glass filaments are linear and the thermoplastic filaments are wavy.
In EP-A-0 616 055, a process for producing a glass/thermoplastic composite strand is also proposed, which consists in mingling a web of thermoplastic filaments with a bundle or a web of glass filaments, the thermoplastic filaments being, upstream of the point of convergence, heated to a temperature above their relaxation temperature, drawn then cooled. The composite strand obtained has no waviness and is stable over time.
The direct manufacture of rovings, without passing through an intermediate step of unwinding the tape and winding the strand, is carried out continuously by drawing the composite strand under the bushing at a speed compatible with the drawing of the glass filaments. This already high speed (of the order of a few meters to about ten meters per second) is associated with a drawing speed of the thermoplastic filaments upstream of the convergence points that is even higher.
The production of a composite strand without waviness under such conditions occurs via a precise synchronization of the relative speeds of the rotating elements of the drawing unit and by maintaining the initial difference between the drawing speeds of the glass filaments and the thermoplastic filaments.
These conditions are limited to thermoplastic materials that undergo a limited shrinkage. When the shrinkage is larger, the drawing unit becomes inoperable due to the fact that its speed can no longer be increased so as to sufficiently increase the length of the thermoplastic filaments so that the composite strand does not have any waviness.